A wide variety of agents are administered to subjects, e.g., to treat diseases or disorders or to provide assistance in diagnosing a disease or disorder. Some potential agents are not appropriate for administration to a subject, e.g., because the agent is very hydrophobic and not able to be formulated in a manner consistent with efficient administration, or because the agent is unstable in a physiologic environment. Other agents can be administered to a subject, but their efficacy would be improved by selective targeting to a particular site in the body. The present invention solves these and other problems.